In order to achieve increased efficiency in gas turbines, higher temperatures are desired in the combustion chamber of the turbine. With the use of such higher temperatures, the walls of the combustion chamber are subjected to thermal stresses and strain. Also, because of economic reasons, it is often desirable to burn heavy residual fuels, which are high in contaminants, rather than pure fuels, which residual fuels add substantially more heat to the combustor chamber walls, such that combustor life and reliability are reduced.
While the use of ceramic combustion chamber walls has been proposed to solve these problems, most combustion chamber walls are still formed from metallic components.
Another solution to solving these problems is to introduce more cooling air to the combustor walls. Such increased air addition, however, has an adverse affect on the temperature distribution pattern of the gases when they are introduced to the turbine blades since there is a large temperature differential between the blade ends where the cooler air flows, and the blade center, which causes serious thermal stress and strain on the blades.
A gas turbine, with improved cooling for the walls of the combustor basket is described in U.S. Pat. No. 3,899,882, which issued to Stephen R. Parker on Aug. 19, 1975 and is assigned to the assignee of the present invention, the contents of said patent being incorporated by reference herein. The combustor described therein has a plurality of combustion air orifices or apertures that are disposed in an annular array about the wall of the combustor. Apertures, known as air scoops, are comprised of a tubular portion, a generally annular flange portion, and an intermediate spacer member that is disposed between the wall of the combustor and the annular flange portion of the air scoop. An arcuate gap is provided on the downstream side of the air scoop that permits the flow of air therethrough and cooling of the combustion basket walls. A tubular portion of the air scoop, which extends radially inwardly into the combustion chamber, forces some of the air into the inner portion of the combustor for combustion of the fuel and mixing of the combustion products.
While the features of the combustion chamber of U.S. Pat. No. 3,899,882 do provide cooling of combustor chamber walls, and introduction of air used to burn fuel, in the combustor basket, a problem is posed by the burning away of the extended tubular portion of the scoop which can lead to costly repairs and customer dissatisfaction. The tubular portion of the scoop burns because of the excessive temperature in an oxidizing atmosphere existing in the combustion chamber. The air that flows through the tubular section of the scoop is unable to keep the metal cool because of local separation. The sharp radius that exists at the connection between the annular flange and the tubular portion of the scoop encourages such separation.
It is an object of the present invention to provide a gas turbine combustion chamber with air scoops through the wall of the chamber which air scoops are provided with cooling and which prevent local separation within the scoop to improve flow control of the air into the combustion chamber.